zubbo-
First, if you're converting because your wood spars didn't pass inspection, I want to offer my sympathy. That happened to me, too.
"Anybody can tell me how to go about converting my 1972 7GCAA wooden spars to metal?"
Use the search feature on this forum and search on "milman". Note that there is only one "L" in Milman. These are the STC'd aluminum spars for our planes. They're awesome, and there's lots of info here.
"The factory wants to sell me new wings but its very expensive. "
I converted to Milman spars for about $10k. The divorce was extra. YMMV.
"Can I buy the metal spars and have a qualified person install them on the existing ribs?"
Yes, but... You have to find someone willing, able and affordable to do the work, plus someone qualified to inspect the work. The first person may end up being you. The second can be a local, friendly IA with fabric experience. It's mostly elbow grease. Help is available for the occasional technical challenge.
Good luck. Keep us posted.
